Computational mechanism underlying switching of motor actions

Abstract

Survival of species in an ever-changing environment requires a flexibility that encompasses not only choosing the most appropriate action but also stopping unwanted or inappropriate actions and, more frequently, rapidly switching between actions. Although considerable research has been devoted to understand how the brain switches actions, the computations underlying the switching process and how it relates to selecting and stopping actions remain elusive. A normative theory suggests that switching is simply an extension of the stopping process, during which a current action is first inhibited by an independent pause mechanism before a new action is generated. This theory was challenged by the affordance competition theory, according to which action selection occurs through a competition between parallel prepared actions, therefore the switching process is also implemented by the same mechanism through a competition between the current and the new action, without engaging a pause mechanism. Our study aims to dissociate between these two theories in order to advance our understanding of the switching process. To do so, we utilized a neurocomputational theory that models the process of selecting, stopping and switching reaching movements. We tested the model predictions in healthy individuals who performed reaches in dynamic and uncertain environments that often require stopping and switching actions. Our findings suggest that unlike the stopping process, switching of actions does not necessitate a proactive pause mechanism to delay movement initiation. Hence, the switching and stopping processes seem to be implemented by different mechanisms at the planning phase of the reaching movement. However, the model predicts that once the reaching movement has been initiated, the switching process engages a pause mechanism if the new target location is unknown prior to movement initiation. These findings offer a new understanding of the computations underlying switching actions, contribute valuable insights into the fundamental neuroscientific mechanisms of action regulation, and open new avenues for future neurophysiological investigations.